1. Field of the Invention
This invention relates generally to state of charge (SOC) balancing among battery packs and, more particularly, to a method and system for power management among battery packs with varying states of health which uses a switched transformer to provide proportional active SOC balancing.
2. Discussion of the Related Art
Electric vehicles and engine/electric hybrid vehicles are rapidly gaining popularity in today's automotive marketplace. Electric and hybrid vehicles offer several desirable features, such as reduced emissions, reduced usage of petroleum-based fuels, and potentially lower operating costs. A key component of both electric and hybrid vehicles is the battery pack. Battery packs in these vehicles typically consist of numerous interconnected cells, which can deliver a lot of power on demand to drive the vehicle.
After a few years of service in an electric or hybrid vehicle, battery packs often need to be replaced due to degradation and variation in the state of health of the individual cells, which leads to reduced driving range for the vehicle. Yet, even with a somewhat reduced state of health, an electric vehicle battery pack can still store a considerable amount of energy, and can be used for other applications besides powering a vehicle. A variety of different post-vehicle-life uses for such battery packs have been proposed, including using the battery packs in Community Energy Storage (CES) systems.
CES systems store energy for a small community, such as a residential subdivision or a commercial or industrial complex. CES systems typically serve to augment power available from the utility grid, and are useful in their ability to store locally-generated energy from sources such as solar and wind. Post-vehicle-life battery packs from electric vehicles can be used in CES systems, but their efficiency may be diminished by variations in the state of health of individual cells or sections of the battery packs. A method of managing variations in battery state of health is needed which does not simply bleed off power from higher state of charge batteries, and which enables use of maximum available energy in the battery packs.